How to you find all world treasures

I believe you need to play through the entire game without shooting anyone to gain this achievement. If you mess up, you can reload your last checkpoint and go again but if you've passed another checkpoint since, you'll need to restart.

If you want to check as you go along, you can press the "Back" button and in the top-right of the screen it should say: "So far, you have not fired a shot at anyone". If you think you may have shot someone, hit the "Back" button and check this. If it's gone, you need to reload your last checkpoint. If it's there, you're good.

The bit with the convoy doesn't count as a shot and you can actually shoot guns in the game (at servers etc), as long as you don't shoot at anyone.

The update notes say:

• Added 4 more achievements along the same lines as the "Medium Rare" achievement added last week
• Renamed "Medium Rare" achievement to "Medium Well"

Since they are along the same lines as the previous achievement, they mean the player should complete both the light world and the dark world of a chapter, and the sum of the best times for that chapter should be lower than some threshold. Here are the descriptions of each one:

• Rare - Spend as little time as possible in The Forest (around 320 seconds)
• Medium Rare - Spend as little time as possible in The Hospital (around 430 or 460 or 490 seconds)
• Medium - Spend as little time as possible in The Salt Factory (around 570 seconds)
• Medium Well - Spend as little time as possible in Hell (around 500 or 550 seconds)
• Well Done - Spend as little time as possible in The Rapture (around 740 seconds or more)

About the old secret achievements:

• Suffragette - Complete the light world of the Cotton Alley
• Seneca Falls - Complete the dark world of the Cotton Alley
• Well look at you! - Input the petaphile code to play with Tofu Boy

Sources:

• Super Meat Boy Wikia
• SteamStats
• Steam Forums

Note about the times for the achievements: Many people use Super Meat Boy Stats (old repo at Google Code, new repo at GitHub) program to find out how much time they have spent on each chapter. However, the reported total time is not exactly accurate because "it counts 15 secs per completed warp zone", and people posting their total times didn't mention if the warp zone time has been subtracted or not. For this reason, the times I posted above might be quite inaccurate.